Combustion Air Ducts, Part II: Problems and Solutions

Last week I wrote about what combustion air ducts are and why houses need them. This week I'm going to follow up with some of the most common problems and solutions related to combustion air duct installation and maintenance.

By far, the most common problem that occurs with combustion air ducts is that they get blocked. When a combustion air duct is blocked, air needs to 'leak' in to the house through many different undesirable pathways. I've done a number of home inspections where the windows were completely iced shut throughout the house, and in every case there was a blocked combustion air duct.

A combustion air duct brings in fresh outdoor air, which usually means cold outdoor air in Minnesota. This can create a cold floor where the duct terminates, as well as a cold draft. I was going to make a nice little drawing of this cold air coming in to the basement around my own combustion air duct, but then I remembered I have an IR camera. Duh. Check out the two images below for a nice visual of how the combustion air duct is making my basement floor cold.

To prevent this cold air from dumping in to their home, people sometimes stuff clothes or towels in to the combustion air duct, or the block the intake at the exterior of the home.

Solution: Remove any obstructions. If you want to help cut down on the amount of cold air that just 'dumps' down in to the basement, try creating a trap at the bottom of the combustion air duct. Make the air have to rise back up again before coming in to the home. I don't have any hardcore proof that this makes a big difference, but I've convinced myself that it helps, and it's easy enough to do. The two most common ways of creating a trap are to either make a "J" at the bottom of the duct, or to put a bucket or box underneath the duct. With either of these methods, the air will need to rise up before coming in to the home.

Just make sure that the bucket or box you use isn't so small that it restricts air flow. I've always just eyeballed this, but if you're super anal, you could make your sixth grade math pay off by measuring the inside diameter of the bucket and the outside diameter of the duct, then calculate the surface areas (Πr²) and make sure the bucket's is at least twice that of the duct's.

Also, make sure the duct isn't so long that the opening sits flat on the floor, effectively blocking it.

The opening at the exterior for the combustion air duct will bring air in to the home, and with that comes dust, dirt, insects, leaves, etc. I've found that the closer the combustion air duct is located to the ground, the more likely it's going to get blocked with debris.

Solution: take a peek underneath your combustion air duct every year to make sure it stays clean. If you do this during the summer or fall, watch out for wasps. They love to make nests in this opening. If the opening is dirty, vacuum it off. If you have an HRV, check the HRV intake at the same time.

The opening at the exterior of the home needs to be covered with a steel mesh having openings not less than 1/4", and not more than 1/2". When standard window screen is used here, it will get dirty very quickly. Click on the photo below for a larger view; you'll see the opening is actually covered with a window screen, which should be removed.

Solution: Remove any restrictive mesh or material, and replace it with 1/4" hardware cloth or something similar if it's not already present.

Every so often, vinyl siding installers will forget which opening was meant for the combustion air intake, and they'll install a damper at this opening instead of a screen. These dampers allow air out, not in.

Solution: Replace the exterior terminal with a type that is designed for a combustion air intake, or remove the damper and cover the opening with 1/4" hardware cloth.

The inlet for the combustion air duct needs to be installed at least 12" above grade. When it's too close to the ground, it can get dirty very quickly, and can get blocked over with snow.

Solution: When the combustion air inlet is installed this close to the ground, it's usually done because that's where the rim joist was located, so making a higher hole in the side of the house isn't an option. The solution is to install what Milind calls a 'snorkel'. I laughed the first time I heard this, but I like this term. I think the photo below is pretty self-explanatory.

That concludes my list of the most common installation and maintenance defects with combustion air ducts. If I think of any more, I'll add them to this list in the future.

Author: Reuben Saltzman, Structure Tech Home Inspections